Many image projection systems, such as might be used for projection televisions, are based on the use of liquid crystal display (LCD) imager panels. Some LCD panels operate in a reflective mode, in which incident illumination light is separated from reflected image light by using a polarizing beamsplitter in front of the LCD panel. In such configurations, the illumination light is passed to the LCD panel via the polarization beamsplitter. The illumination light incident at the LCD panel is, therefore, polarized. The LCD panel operates by selectively adjusting the polarization modulation of the many pixels of the panel. Those pixels associated with dark areas of the image do not alter the polarization state of the light whereas those pixels associated with bright areas of the image do alter the polarization state of the light. When the illumination light is reflected to the LCD panel by the polarization beamsplitter as reflected light, that light which has polarization that has been changed to the polarization state that is orthogonal to the polarization state of the incident light is transmitted through the polarization beamsplitter. Ideally, only the light corresponding to pixels that actively modulate the incident light is transmitted through the polarization beamsplitter to the projector's lens system, while light reflected by pixels that correspond to dark areas of the image, i.e. pixels that are not actively modulating the light, is rejected by the polarization beamsplitter. Thus, the beamsplitter can be used to separate the polarization modulated image light from the unmodulated light, which results in an image beam that can be projected.
An important characteristic of a display is the ANSI contrast ratio, as described in: Electronic Projection—Fixed Resolution Projectors”, ANSI/NAPM IT7.228-1997, which is a measure of the maximum brightness of one pixel of the display relative to the minimum brightness of an adjacent pixel. Any “crossover” of light from a bright pixel to an adjacent dark pixel, for example resulting from reflection of light within the system, results in a reduction in ANSI contrast, and thus the quality of the viewed image is reduced. ANSI contrast is one means of characterizing the presence of stray light that is intended to illuminate a bright area of an image, but is redirected by scattering or unintended reflections into a nominally dark region of the image. Because it measures the stray light at the center of large dark regions (as described in the ANSI standard) it is not sensitive to light that is redirected only onto the edges of the nominally dark regions.